In recent years, substantial progress has been made in the development of techniques for providing effective medical response to various heart disorders or arrhythmias. The types of contemplated disorders or arrhythmias have typically been treated in the past by drug therapy, or by devices such as pacers, defibrillators, cardioverters, etc.
More recent efforts have resulted in the development of electronic standby defibrillators, such as disclosed in U.S. Pat. No. Re. 27,652 of Mirowski et al (based on original U.S. Pat. No. 3,614,954) and U.S. Pat. No. Re. 27,757 of Mirowski et al (based on original U.S. Pat. No. 3,614,955).
Most recently, efforts have been directed toward the development of miniaturized defibrillating, cardioverting and pacing devices amenable to implantation in the body of a patient subject to heart disorder or arrhythmia. An example of one such implantable device is contained in U.S. Pat. No. 3,952,750 of Mirowski et al (which discloses a command atrial cardioverting device). The utilization of an implantable automatic defibrillator is referred to in U.S. Pat. No. 4,030,509 to Heilman et al. Moreover, U.S. Pat. No. 4,164,946 to Langer discloses a fault detection circuit for a permanently implanted cardioverter.
Despite the developments of the recent past, there remains much room for advancement in this area of medical technology. For example, it is considered highly desirable to develop a single implantable heart stimulator having the capability of selectively performing any one of the various techniques for responding medically to recognizable heart disorders or arrhythmias, that is to say, the development of a single implanted heart stimulator capable of performing defibrillating, cardioverting, and pacing functions on a selective basis.
It also is highly desirable to develop an implantable heart stimulator and related method capable of selectively performing any one of these techniques on an automatic basis, that is, automatically in response to detection of the occurrence of the corresponding heart disorder or arrhythmia.
Moreover, an extremely advantageous feature of such a device and method would reside in the capability of externally programming the device to perform various operations, or sequences of operations, in accordance with defined parameters. Further elaboration on this point, including a background discussion, is appropriate at this point.
It is known that the human heart requires coordinated electrical activity to successfully supply the body with a sufficient flow of blood. This coordinated activity is produced by a specialized conduction system contained in the body. A description of this system can be seen by reference to The CIBA Collection of Medical Illustrations, Heart, by Frank Netter, M.D., pp. 49-49, 1974 (ISBN 0-914168-07-X, Library of Congress Catalog No. 53-2151). Malfunctions of the conduction system produce a variety of human disease conditions up to and including death (see Netter, op. cit., pp. 66-68).
Recently, an implantable automatic defibrillator has been developed. The defibrillator automatically delivers a large electrical pulse to the fibrillating ventricles to abolish fatal malfunction and, thus, may be lifesaving in the case of ventricular fibrillation. Moreover, numerous other forms of electrical stimulation therapy have been, and are being, developed to treat various abnormalities of the heart.
For example, it is known that asystole (the absence of electrical stimulation to the ventricles of the heart) may be treated by implanting a pacer which periodically stimulates the ventricles with an electrical pacing pulse. Moreover, sophisticated pacing techniques, including various pacing modes (to be discussed in more detail below), have been developed.
Most automatic devices provide pulses to the atrium of the heart, but practitioners are reluctant to automatically treat the ventricle of the heart by pacing because of the dangers involved, for example, induced fibrillation. Accordingly, it is considered desirable to develop a device which has the capability of treating, by pacing modes and, if need be by backup defibrillation, any induced arrhythmia or fibrillation which might result from treatment of the ventricle of the heart.
It is presently known that electrical stimulation treatment modalities may be primarily classified in accordance with the energy level utilized, as follows:
______________________________________ Pulse Type Energy Range ______________________________________ Pacing equal to or less than 100 microjoules Cardioverting or 1-100 joules defibrillating (internal) ______________________________________
Stated in simple terms, pacing pulses stimulate a very small volume of heart tissue (approximately 1-10 mm.sup.3), and the impulse is then contiguously conducted in a spreading fashion. Defibrillating pulses, on the other hand, are of sufficient strength to simultaneously stimulate all, or a critical mass, of the heart tissue, thus ameloriating the dangerous disorganized patterns of cyclic self-stimulation associated with ventricular fibrillation.
In the very recent past, combined pacing and cardioverting electrode systems have been developed, such as are described in U.S. Pat. No. 4,030,509 noted above. Such systems allow the delivery of defibrillating energy to either the atria or ventricles, and also allow for the delivery of pacing pulses. A large number of possible electrical stimulation options are thereby made possible from the combined electrodes.
Such combined pacing and defibrillating functions are quite effective in an implanted device because some symptoms, such as the absence of R-waves, could indicate an asystole (treatable by pacing) or life-threatening ventricular fibrillation. It therefore would be desirable to have a combined pacer-defibrillator that first could attempt pacing in the presence of such symptons, and then, if the symptons persist, attempt defibrillation.
A further copending patent application, Ser. No. 902,763 of Langer et al, is directed toward the development of a data recording device, intended for implantation along with an implantable automatic defibrillator. The intended purpose is to record approximately 100 seconds of the heart's electrogram before, during and after an episode of ventricular fibrillation. At a later time, the stored information may be extracted to provide a complete, permanent record of the ventricular fibrillation episode, including the operation of the device during automatic defibrillation. The use of this recording capability may be extended to capture critical data for additional modes of electrical stimulation therapy, and also to gain information which could lead to more effective future electrical stimulation.
Pacers are increasingly becoming programmable, whereby parameters such as pulse rate, pulse amplitude and R-wave sensitivity may be adjusted from an external device in electromagnetic communication with the implanted pacer. It would be highly desirable to implant a microprocessor within an implanted pacer/cardioverter, for a communication link could thus be established to enter data, such as a new program, changing the software program (and, hence, operation) of the microprocessor. Moreover, the presence of a microprocessor would allow the use of extensive logic and analysis in the diagnosis and treatment of heart malfunctions with various regimens of electrical heart stimulation. Thus, it is considered highly desirable to develop an implantable heart stimulator capable of performing more than one mode of electrical heart stimulation for a given malfunction, and further provided with the capability of utilizing a variety of parameters within any given mode of operation, and even further with the capability of employing logic in a variety of fashions.
Further referring to the employment of a microprocessor in an implantable heart stimulator, it is to be recognized that various microprocessors available today vary in both power consumption and speed, thus making certain microprocessors (of lower power and speed) suitable for long-term operations, while other microprocessors (of higher power and speed) are more suitable for performance of sophisticated operations on a short-term basis. Accordingly, it is considered highly desirable to provide an implanted heart stimulator with a dual processor capability. It is also desirable to provide the heart stimulator with both a high power, high speed processor and a low power, low speed processor. This would especially be advantageous in view of the further design criterion of providing an implantable heart stimulator having multiple modes of operation for performing various electrical heart stimulation techniques (as discussed above), since some operations would be suitable for performance by one processor, while others would be more suitable for performance by the other microprocessor.
Finally, there are times, during operation, when it would be preferable for a given processor to operate at a speed higher than its normal speed of operation. Thus, it is considered highly desirable for an implantable, microprocessor-based heart stimulator to have the built-in capability of "gear shifting" so that the microprocessor operates temporarily at a higher speed of operation.